This invention relates to coatings for enhancing the solute rejection of reverse osmosis membranes. More particularly, this invention pertains to a complex formed from a cationic material and an anionic material which is useful for restoring or enhancing solute rejection of reverse osmosis membranes.
Semipermeable membranes are fabricated from a variety of materials and in diverse configurations. Polyamide and cellulose ester discriminating layers are widely employed in commercial reverse osmosis membranes. The membranes can be used in tubular, spiral, flat sheet or hollow fiber devices. Composite, asymmetric and homogeneous membranes are the principal forms used.
A single membrane which displays excellent chemical stability, high flux, good solute rejection and resistance to fouling has long been sought. Generally, outstanding performance in one aspect has only been achieved with compromises in other areas of performance. Moreover, both rejection and flux of a membrane tend to decline over time.
Polyelectrolyte complexes have been used in the prior art as dialysis membranes. U.S. Pat. No. 3,276,598 discloses that these films exhibit very high diffusivity to sodium chloride and other simple electrolytes, but very low diffusivity toward higher molecular weight compounds or ionic species.
A coating system which enhances or restores performance of reverse osmosis membranes has long been sought. U.S. Pat. No. 3,877,978 describes the use of certain vinyl acetate copolymers to improve rejection of membranes. U.S. Pat. No. 3,886,066 discloses the use of hydrolyzable tannins to reduce solute passage of reverse osmosis membranes. U.S. Pat. No. 4,214,020 claims a process for coating membranes assembled in devices.